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On proof-test intervals for safety functions implemented in software
Griffiths, A.
Given a target probability of functional failure on demand
for a system, a corresponding dangerous failure rate for the
system can be derived, provided that a proof-test interval
for the function is known. IEC 61508, and related
standards, requires that this calculation be performed, for
certain kinds of systems that are required to provide safety
functionality on demand. This paper explains why it is
necessary to consider what constitutes a proof-test interval
for a function, and then considers what this means for
software. We show that there are several problems with the
proof-test concept, as applied to software, and describe the
problems this presents to practitioners wanting to derive
safety integrity levels for system safety functions.
Cite as: Griffiths, A. (2006). On proof-test intervals for safety functions implemented in software. In Proc. Eleventh Australian Workshop on Safety-Related Programmable Systems (SCS 2006), Melbourne, Australia. CRPIT, 69. Cant, T., Ed. ACS. 23-33.
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